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Modulo VI
Sistemas de
Entrada e Saída
Prof. Ismael H F Santos
April 05
Prof. Ismael H. F. Santos - [email protected]
1
Ementa
 Gerência de Entrada e Saída – Armazenamento
 Disk Structure
 Disk Scheduling
 Disk Management
 RAID Structure
 Tertiary Storage Devices
 Operating System Issues
 Gerência de Entrada e Saída – Implementação
 I/O Hardware - DMA
 Application I/O Interface
 Kernel I/O Subsystem
April 05
Prof. Ismael H. F. Santos - [email protected]
2
SOP – CO023
Gerência de
E/S
April 05
Prof. Ismael H. F. Santos - [email protected]
3
Objectives
 Describe the physical structure of secondary
and tertiary storage devices and the resulting
effects on the uses of the devices
 Explain the performance characteristics of
mass-storage devices
 Discuss operating-system services provided
for mass storage, including RAID and HSM
April 05
Prof. Ismael H. F. Santos - [email protected]
4
Overview of Mass Storage Structure
 Magnetic disks provide bulk of secondary storage of
modern computers




April 05
Drives rotate at 60 to 200 times per second
Transfer rate is rate at which data flow between drive
and computer
Positioning time (random-access time) is time to
move disk arm to desired cylinder (seek time) and
time for desired sector to rotate under the disk head
(rotational latency)
Head crash results from disk head making contact
with the disk surface
 That’s bad
Prof. Ismael H. F. Santos - [email protected]
5
Overview of Mass Storage Structure
 Disks can be removable
 Drive attached to computer via I/O bus
 Busses vary, including EIDE, ATA, SATA, USB, Fibre
Channel, SCSI
 Host controller in computer uses bus to talk to disk
controller built into drive or storage array
April 05
Prof. Ismael H. F. Santos - [email protected]
6
Moving-head Disk Machanism
April 05
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Overview of Mass Storage Structure
(Cont.)
 Magnetic tape
 Was early secondary-storage medium
 Relatively permanent and holds large quantities of data
 Access time slow
 Random access ~1000 times slower than disk
 Mainly used for backup, storage of infrequently-used data,
transfer medium between systems
 Kept in spool and wound or rewound past read-write head
 Once data under head, transfer rates comparable to disk
 20-200GB typical storage
 Common technologies are 4mm, 8mm, 19mm, LTO-2 and
SDLT
April 05
Prof. Ismael H. F. Santos - [email protected]
8
SOP – CO023
April 05
Estrutura
Do
Disco
Prof. Ismael H. F. Santos - [email protected]
9
Disk Structure
 Disk drives are addressed as large 1-dimensional
arrays of logical blocks, where the logical block is the
smallest unit of transfer.
 The 1-dimensional array of logical blocks is mapped
into the sectors of the disk sequentially.
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
April 05
Sector 0 is the first sector of the first track on the
outermost cylinder.
Mapping proceeds in order through that track, then the
rest of the tracks in that cylinder, and then through the
rest of the cylinders from outermost to innermost.
Prof. Ismael H. F. Santos - [email protected]
10
Disk Attachment
 Host-attached storage accessed through I/O ports
talking to I/O busses
 SCSI itself is a bus, up to 16 devices on one cable,
SCSI initiator requests operation and SCSI targets
perform tasks

Each target can have up to 8 logical units (disks
attached to device controller
 FC is high-speed serial architecture
 Can be switched fabric with 24-bit address space – the
basis of storage area networks (SANs) in which many
hosts attach to many storage units
 Can be arbitrated loop (FC-AL) of 126 devices
April 05
Prof. Ismael H. F. Santos - [email protected]
11
Network-Attached Storage
 Network-attached storage (NAS) is storage made
available over a network rather than over a local
connection (such as a bus)
 NFS and CIFS are common protocols
 Implemented via remote procedure calls (RPCs)
between host and storage
 New iSCSI
protocol uses
IP network to
carry the SCSI
protocol
April 05
Prof. Ismael H. F. Santos - [email protected]
12
Storage Area Network
 Common in large storage environments (and
becoming more common)
 Multiple hosts attached to multiple storage arrays flexible
April 05
Prof. Ismael H. F. Santos - [email protected]
13
SOP – CO023
April 05
Escalonamento
Do
Disco
Prof. Ismael H. F. Santos - [email protected]
14
Disk Scheduling
 The operating system is responsible for using
hardware efficiently — for the disk drives, this means
having a fast access time and disk bandwidth.
 Access time has two major components


Seek time is the time for the disk are to move the
heads to the cylinder containing the desired sector.
Rotational latency is the additional time waiting for the
disk to rotate the desired sector to the disk head.
 Minimize seek time
 Seek time  seek distance
April 05
Prof. Ismael H. F. Santos - [email protected]
15
Disk Scheduling (Cont.)
 Disk bandwidth is the total number of bytes
transferred, divided by the total time between the first
request for service and the completion of the last
transfer.
 Several algorithms exist to schedule the servicing of
disk I/O requests. We illustrate them with a request
queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
April 05
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FCFS
Total head movement of 640 cylinders.
April 05
Prof. Ismael H. F. Santos - [email protected]
17
SSTF
 Selects the request
with the minimum
seek time from the
current head
position.
 SSTF scheduling is
a form of SJF
scheduling; may
cause starvation of
some requests.
 Total head
movement of 236
cylinders !
April 05
Prof. Ismael H. F. Santos - [email protected]
18
SCAN
 The disk arm starts at
one end of the disk, and
moves toward the other
end, servicing requests
until it gets to the other
end of the disk, where
the head movement is
reversed and servicing
continues.
 Sometimes called the
elevator algorithm.
 Total head movement
of 208 cylinders !
April 05
Prof. Ismael H. F. Santos - [email protected]
19
C-SCAN
 Provides a more uniform wait time than SCAN.
 The head moves from one end of the disk to
the other servicing requests as it goes. When
it reaches the other end, however, it
immediately returns to the beginning of the
disk, without servicing any requests on the
return trip.
 Treats the cylinders as a circular list that wraps
around from the last cylinder to the first one.
April 05
Prof. Ismael H. F. Santos - [email protected]
20
C-SCAN (Cont.)
April 05
Prof. Ismael H. F. Santos - [email protected]
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C-LOOK
 Version of C-SCAN
 Arm only goes as
far as the last
request in each
direction, then
reverses direction
immediately, without
first going all the
way to the end of
the disk.
April 05
Prof. Ismael H. F. Santos - [email protected]
22
Selecting a Disk-Scheduling
Algorithm
 SSTF is common and has a natural appeal
 SCAN and C-SCAN perform better for systems that
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
place a heavy load on the disk.
Performance depends on the number and types of
requests.
Requests for disk service can be influenced by the fileallocation method.
The disk-scheduling algorithm should be written as a
separate module of the operating system, allowing it to
be replaced with a different algorithm if necessary.
Either SSTF or C-LOOK is a reasonable choice for the
default algorithm.
April 05
Prof. Ismael H. F. Santos - [email protected]
23
SOP – CO023
April 05
Gerenciamento
Do
Disco
Prof. Ismael H. F. Santos - [email protected]
24
Disk Management
 Low-level formatting, or physical formatting — Dividing a
disk into sectors that the disk controller can read and
write.
 To use a disk to hold files, the operating system still
needs to record its own data structures on the disk.


Partition the disk into one or more groups of cylinders.
Logical formatting or “making a file system”.
 Boot block initializes system.
 The bootstrap is stored in ROM.
 Bootstrap loader program.
 Methods such as sector sparing used to handle bad
blocks.
April 05
Prof. Ismael H. F. Santos - [email protected]
25
Booting from a Disk in Windows
2000
April 05
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Swap-Space Management
 Swap-space — Virtual memory uses disk space as an
extension of main memory. Can be carved out of the
normal file system, or, more commonly, it can be in a
separate disk partition.
 Swap-space management
 4.3BSD allocates swap space when process starts; holds
text segment (the program) and data segment.
 Kernel uses swap maps to track swap-space use.
 Solaris 2 allocates swap space only when a page is forced
out of physical memory, not when the virtual memory page
is first created.
April 05
Prof. Ismael H. F. Santos - [email protected]
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Data Structures for Swapping on
Linux Systems
April 05
Prof. Ismael H. F. Santos - [email protected]
28
SOP – CO023
RAID
April 05
Prof. Ismael H. F. Santos - [email protected]
29
RAID Structure
 RAID – multiple disk drives provides reliability via
redundancy. RAID is arranged into six different levels.
 Several improvements in disk-use techniques involve
the use of multiple disks working cooperatively.
 Disk striping uses a group of disks as one storage unit.
 RAID schemes improve performance and improve the
reliability of the storage system by storing redundant
data.
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
April 05
Mirroring or shadowing keeps duplicate of each disk.
Block interleaved parity uses much less redundancy.
Prof. Ismael H. F. Santos - [email protected]
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RAID Levels
April 05
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31
RAID (0 + 1) and (1 + 0)
April 05
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Stable-Storage Implementation
 Write-ahead log scheme requires stable
storage.
 To implement stable storage:
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
April 05
Replicate information on more than one
nonvolatile storage media with independent
failure modes.
Update information in a controlled manner to
ensure that we can recover the stable data
after any failure during data transfer or
recovery.
Prof. Ismael H. F. Santos - [email protected]
33
SOP – CO023
April 05
Memórias
De
Massa
Prof. Ismael H. F. Santos - [email protected]
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Tertiary Storage Devices
 Low cost is the defining characteristic of
tertiary storage.
 Generally, tertiary storage is built using
removable media
 Common examples of removable media are
floppy disks and CD-ROMs; other types are
available.
April 05
Prof. Ismael H. F. Santos - [email protected]
35
Removable Disks
 Floppy disk — thin flexible disk coated with
magnetic material, enclosed in a protective
plastic case.


April 05
Most floppies hold about 1 MB; similar
technology is used for removable disks that
hold more than 1 GB.
Removable magnetic disks can be nearly as
fast as hard disks, but they are at a greater risk
of damage from exposure.
Prof. Ismael H. F. Santos - [email protected]
36
Removable Disks (Cont.)
 A magneto-optic disk records data on a rigid
platter coated with magnetic material.
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
April 05
Laser heat is used to amplify a large, weak
magnetic field to record a bit.
Laser light is also used to read data (Kerr
effect).
The magneto-optic head flies much farther
from the disk surface than a magnetic disk
head, and the magnetic material is covered
with a protective layer of plastic or glass;
resistant to head crashes.
Prof. Ismael H. F. Santos - [email protected]
37
WORM Disks
 The data on read-write disks can be modified over and
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



over.
WORM (“Write Once, Read Many Times”) disks can be
written only once.
Thin aluminum film sandwiched between two glass or
plastic platters.
To write a bit, the drive uses a laser light to burn a small
hole through the aluminum; information can be
destroyed by not altered.
Very durable and reliable.
Read Only disks, such ad CD-ROM and DVD, com
from the factory with the data pre-recorded.
April 05
Prof. Ismael H. F. Santos - [email protected]
38
Tapes
 Compared to a disk, a tape is less expensive and holds
more data, but random access is much slower.
 Tape is an economical medium for purposes that do not
require fast random access, e.g., backup copies of disk
data, holding huge volumes of data.
April 05
Prof. Ismael H. F. Santos - [email protected]
39
Tapes (cont.)
 Large tape installations typically use robotic tape
changers that move tapes between tape drives and
storage slots in a tape library.

stacker – library that holds a few tapes

silo – library that holds thousands of tapes
 A disk-resident file can be archived to tape for low
cost storage; the computer can stage it back into disk
storage for active use.
April 05
Prof. Ismael H. F. Santos - [email protected]
40
Operating System Issues
 Major OS jobs are to manage physical
devices and to present a virtual machine
abstraction to applications
 For hard disks, the OS provides two
abstraction:


April 05
Raw device – an array of data blocks.
File system – the OS queues and schedules
the interleaved requests from several
applications.
Prof. Ismael H. F. Santos - [email protected]
41
Application Interface
 Most OSs handle removable disks almost exactly like
fixed disks — a new cartridge is formatted and an
empty file system is generated on the disk.
 Tapes are presented as a raw storage medium, i.e.,
and application does not not open a file on the tape, it
opens the whole tape drive as a raw device.
 Usually the tape drive is reserved for the exclusive use
of that application.
April 05
Prof. Ismael H. F. Santos - [email protected]
42
Application Interface (cont.)
 Since the OS does not provide file system services, the
application must decide how to use the array of blocks.
 Since every application makes up its own rules for how
to organize a tape, a tape full of data can generally
only be used by the program that created it.
April 05
Prof. Ismael H. F. Santos - [email protected]
43
Tape Drives
 The basic operations for a tape drive differ from those





of a disk drive.
locate positions the tape to a specific logical block, not
an entire track (corresponds to seek).
The read position operation returns the logical block
number where the tape head is.
The space operation enables relative motion.
Tape drives are “append-only” devices; updating a
block in the middle of the tape also effectively erases
everything beyond that block.
An EOT mark is placed after a block that is written.
April 05
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44
File Naming
 The issue of naming files on removable media is
especially difficult when we want to write data on a
removable cartridge on one computer, and then use the
cartridge in another computer.
 Contemporary OSs generally leave the name space
problem unsolved for removable media, and depend on
applications and users to figure out how to access and
interpret the data.
 Some kinds of removable media (e.g., CDs) are so well
standardized that all computers use them the same
way.
April 05
Prof. Ismael H. F. Santos - [email protected]
45
Hierarchical Storage Management
(HSM)
 A hierarchical storage system extends the storage
hierarchy beyond primary memory and secondary
storage to incorporate tertiary storage — usually
implemented as a jukebox of tapes or removable
disks.
 Usually incorporate tertiary storage by extending the
file system.


Small and frequently used files remain on disk.
Large, old, inactive files are archived to the jukebox.
 HSM is usually found in supercomputing centers and
other large installations that have enormous volumes
of data.
April 05
Prof. Ismael H. F. Santos - [email protected]
46
Speed
 Two aspects of speed in tertiary storage are
bandwidth and latency.
 Bandwidth is measured in bytes per second.
 Sustained bandwidth – average data rate during a
large transfer; # of bytes/transfer time.
Data rate when the data stream is actually flowing.

April 05
Effective bandwidth – average over the entire I/O time,
including seek or locate, and cartridge switching.
Drive’s overall data rate.
Prof. Ismael H. F. Santos - [email protected]
47
Speed (Cont.)
 Access latency – amount of time needed to locate
data.



April 05
Access time for a disk – move the arm to the selected
cylinder and wait for the rotational latency; < 35
milliseconds.
Access on tape requires winding the tape reels until the
selected block reaches the tape head; tens or hundreds
of seconds.
Generally say that random access within a tape cartridge
is about a thousand times slower than random access on
disk.
Prof. Ismael H. F. Santos - [email protected]
48
Speed (Cont.)
 The low cost of tertiary storage is a result of having
many cheap cartridges share a few expensive drives.
 A removable library is best devoted to the storage of
infrequently used data, because the library can only
satisfy a relatively small number of I/O requests per
hour.
April 05
Prof. Ismael H. F. Santos - [email protected]
49
Reliability
 A fixed disk drive is likely to be more reliable
than a removable disk or tape drive.
 An optical cartridge is likely to be more
reliable than a magnetic disk or tape.
 A head crash in a fixed hard disk generally
destroys the data, whereas the failure of a
tape drive or optical disk drive often leaves
the data cartridge unharmed.
April 05
Prof. Ismael H. F. Santos - [email protected]
50
SOP – CO023
Custo
April 05
Prof. Ismael H. F. Santos - [email protected]
51
Cost
 Main memory is much more expensive than disk
storage
 The cost per megabyte of hard disk storage is
competitive with magnetic tape if only one tape is
used per drive.
 The cheapest tape drives and the cheapest disk
drives have had about the same storage capacity
over the years.
 Tertiary storage gives a cost savings only when the
number of cartridges is considerably larger than the
number of drives.
April 05
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52
Price per Megabyte of DRAM, From 1981
to 2004
April 05
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53
Price per Megabyte of Magnetic Hard Disk, From
1981 to 2004
April 05
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54
Price per Megabyte of a Tape Drive, From
1984-2000
April 05
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55
SOP – CO023
Gerência de
E/S
April 05
Prof. Ismael H. F. Santos - [email protected]
56
Objectives
 Explore the structure of an operating system’s
I/O subsystem
 Discuss the principles of I/O hardware and its
complexity
 Provide details of the performance aspects of
I/O hardware and software
April 05
Prof. Ismael H. F. Santos - [email protected]
57
I/O Hardware
 Incredible variety of I/O devices
 Common concepts



Port
Bus (daisy chain or shared direct access)
Controller (host adapter)
 I/O instructions control devices
 Devices have addresses, used by


April 05
Direct I/O instructions
Memory-mapped I/O
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58
A Typical PC Bus Structure
April 05
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59
Device I/O Port Locations on PCs
(partial)
April 05
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60
Polling
 Determines state of device



command-ready
busy
Error
 Busy-wait cycle to wait for I/O from device
April 05
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61
Interrupts
 CPU Interrupt-request line triggered by I/O device
Interrupt handler receives interrupts
 Maskable to ignore or delay some interrupts
 Interrupt vector to dispatch interrupt to correct
handler


Based on priority
Some nonmaskable
 Interrupt mechanism also used for exceptions
April 05
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62
Interrupt-Driven I/O Cycle
April 05
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63
Intel Pentium Processor Event-Vector
Table
April 05
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64
SOP – CO023
DMA
April 05
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65
Direct Memory Access
 Used to avoid programmed I/O for large data
movement
 Requires DMA controller
 Bypasses CPU to transfer data directly
between I/O device and memory
April 05
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66
Six Step Process to Perform DMA
Transfer
April 05
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67
SOP – CO023
April 05
API
IO
Iterface
Prof. Ismael H. F. Santos - [email protected]
68
Application I/O Interface
 I/O system calls encapsulate device
behaviors in generic classes
 Device-driver layer hides differences among
I/O controllers from kernel
 Devices vary in many dimensions





April 05
Character-stream or block
Sequential or random-access
Sharable or dedicated
Speed of operation
read-write, read only, or write only
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69
A Kernel I/O Structure
April 05
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70
Characteristics of I/O Devices
April 05
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71
Block and Character Devices
 Block devices include disk drives
 Commands include read, write, seek
 Raw I/O or file-system access
 Memory-mapped file access possible
 Character devices include keyboards, mice, serial ports
 Commands include get, put

April 05
Libraries layered on top allow line editing
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72
Network Devices
 Varying enough from block and character to have own
interface
 Unix and Windows NT/9x/2000 include socket interface
 Separates network protocol from network operation
 Includes select functionality
 Approaches vary widely (pipes, FIFOs, streams,
queues, mailboxes)
April 05
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73
Clocks and Timers
 Provide current time, elapsed time, timer
 Programmable interval timer used for
timings, periodic interrupts
 ioctl (on UNIX) covers odd aspects of I/O
such as clocks and timers
April 05
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74
Blocking and Nonblocking I/O
 Blocking - process suspended until I/O completed
 Easy to use and understand
 Insufficient for some needs
 Nonblocking - I/O call returns as much as available
 User interface, data copy (buffered I/O)
 Implemented via multi-threading
 Returns quickly with count of bytes read or written
 Asynchronous - process runs while I/O executes
 Difficult to use
 I/O subsystem signals process when I/O completed
April 05
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75
Two I/O Methods
Synchronous
April 05
Asynchronous
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76
SOP – CO023
April 05
Kernel
IO
Subsystem
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77
Kernel I/O Subsystem
 Scheduling
 Some I/O request ordering via per-device queue
 Some OSs try fairness
 Buffering - store data in memory while transferring
between devices



April 05
To cope with device speed mismatch
To cope with device transfer size mismatch
To maintain “copy semantics”
Prof. Ismael H. F. Santos - [email protected]
78
Device-status Table
April 05
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79
Sun Enterprise 6000 Device-Transfer
Rates
April 05
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80
Kernel I/O Subsystem
 Caching - fast memory holding copy of data
 Always just a copy
 Key to performance
 Spooling - hold output for a device
 If device can serve only one request at a time
 i.e., Printing
 Device reservation - provides exclusive access to a
device


April 05
System calls for allocation and deallocation
Watch out for deadlock
Prof. Ismael H. F. Santos - [email protected]
81
Error Handling
 OS can recover from disk read, device
unavailable, transient write failures
 Most return an error number or code when
I/O request fails
 System error logs hold problem reports
April 05
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82
I/O Protection
 User process may accidentally or purposefully
attempt to disrupt normal operation via illegal
I/O instructions


All I/O instructions defined to be privileged
I/O must be performed via system calls

April 05
Memory-mapped and I/O port memory locations
must be protected too
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83
Use of a System Call to Perform
I/O
April 05
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84
Kernel Data Structures
 Kernel keeps state info for I/O components,
including open file tables, network
connections, character device state
 Many, many complex data structures to track
buffers, memory allocation, “dirty” blocks
 Some use object-oriented methods and
message passing to implement I/O
April 05
Prof. Ismael H. F. Santos - [email protected]
85
UNIX I/O Kernel Structure
April 05
Prof. Ismael H. F. Santos - [email protected]
86
I/O Requests to Hardware
Operations
 Consider reading a file from disk for a
process:
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Determine device holding file
Translate name to device representation
Physically read data from disk into buffer
Make data available to requesting process
Return control to process
Prof. Ismael H. F. Santos - [email protected]
87
Life Cycle of An I/O Request
April 05
Prof. Ismael H. F. Santos - [email protected]
88
STREAMS
 STREAM – a full-duplex communication channel
between a user-level process and a device in Unix
System V and beyond
 A STREAM consists of:
- STREAM head interfaces with the user process
- driver end interfaces with the device
- zero or more STREAM modules between them.
 Each module contains a read queue and a write queue.
Message passing is used to communicate between
queues
April 05
Prof. Ismael H. F. Santos - [email protected]
89
The STREAMS Structure
April 05
Prof. Ismael H. F. Santos - [email protected]
90
Performance
 I/O a major factor in system performance:
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Demands CPU to execute device driver,
kernel I/O code
Context switches due to interrupts
Data copying
Network traffic especially stressful
Prof. Ismael H. F. Santos - [email protected]
91
Intercomputer Communications
April 05
Prof. Ismael H. F. Santos - [email protected]
92
Improving Performance
 Reduce number of context switches
 Reduce data copying
 Reduce interrupts by using large transfers,
smart controllers, polling
 Use DMA
 Balance CPU, memory, bus, and I/O
performance for highest throughput
April 05
Prof. Ismael H. F. Santos - [email protected]
93
Device-Functionality Progression
April 05
Prof. Ismael H. F. Santos - [email protected]
94